I traveled to Vienna from Germany through flixbus showing my Aufenthaltstitels at the border. Unfortunately I lost it somewhere in the bus or afterwards. I registered at all lost and founds in Vienna without success. Biggest mistake I reckon I have done is traveling only with a photocopy of my Indian passport (fearing that I would lose the original which is in Germany now). Now the Deutsche botschaft refuses to help me without my passport, and I am out of ideas. Is there a way out of this?
Immigration checks are currently relatively common when entering Germany from Austria by land and even with your Aufenhaltstitel, I would expect that you are likely to experience unnecessary hazzle on the German border, if you try to enter without proper travel documents. The Aufenhaltstitel card is not valid as an id or travel document and would have been required in addition to your passport (and not only a copy of it) to prove your right to stay in the Schengen area. Any letters from the Indian embassy are not recognized as travel documents by the German authorities. If you end up being confronted by a strict immigration officer, you need, at least by the letter of the immigration law, a proper or emergency passport.
Your easiest way out is probably, as Douglas Held suggested, to have your passport brought by a friend or sent by mail/courier to Vienna. With your proper passport, you could then go to the German embassy in Vienna and get a confirmation of your immigration status before travelling back to Germany.